1. Field of the Invention
The invention relates to a field emitter beam source to emit a beam current and a method for controlling a beam current. The invention also relates to an array of field emitter beam sources.
2. Description of the Related Art
Field emitter beam sources are devices to generate beams of electrons for applications like electron microscopy, electron beam pattern generators and display technology. A field emitter beam source emits electrons at the tip of a needle (emitter tip) due to a voltage applied between the emitter tip and an extraction electrode. With a small radius of the emitter tip, even a moderate voltage between the emitter tip and the extracting electrode creates an electric field at the emitter tip that is large enough to extract electrons from the emitter tip into vacuum to form an electron beam.
It has been predicted that field emitter beam sources will replace thermal electron beam sources in many applications due to their higher brightness and smaller spot size. However, field emitter beam sources are disadvantageous in that their beam current stability is poor compared to thermal electron beam emitters. The reason for the beam instability of field emitters is that the radius of the emitter tip is usually so small (typically a few tens of nanometers or less) that the electric field at the tip of the emitters varies significantly when the surface of the emitter tip becomes contaminated or changes due to chemical or physical processes during operation. As a consequence, the emitter beam current fluctuates significantly even if the voltage between extracting electrode and emitter is kept constant. However, for most electron beam devices it is essential to have a controlled electron beam exposure of a specimen in order to obtain an even patterning of the specimen, or a good image contrast when the specimen is inspected.
It is one of the advantages of field emitter beam sources over conventional thermal emitters like tungsten hairpin filaments that field emitter beam sources can be fabricated in arrays by using micromechanical processing techniques. Arrays of integrated field emitter beam sources are also known as micro field emitter arrays. Such arrays may have thousands or even millions of emitter tips with a pitch in the range of only a few micrometers or below. Using large arrays of integrated field emitter beam sources is particularly promising in the field of electron beam lithography. While electron beam lithography offers a potential for much higher spatial resolution capabilities than, e.g., masking technology, it presently suffers from low throughput due to the lengthy process of one beam “writing” a structure. With multiple electron beams in parallel however, electron beam lithography can achieve a throughput which one day may enable it to replace present masking technology.
However, the requirement to stabilize the beam currents of not only one but of a large array of field emitter beam sources poses even larger problems since each field emitter beam source has a different current-voltage behavior due to the high sensitivity to fabrication irregularities. Further, for each field emitter beam source, the current-voltage behavior may change over time which makes it even harder to provide an even electron exposure over the specimen. One way to improve the beam current stability is to control the beam current electronically by using a current source circuit for each emitter. A current source is capable of providing a constant beam current independent of a contamination or deformation of the emitter, since with a current source, the voltage between the extracting electrode and the emitter is free to adjust itself to a value where the current tunneling through the emitter surface-vacuum barrier (Schottky-Barrier) matches the current provided by the current source. Current source circuits that control the current through a field emitter are known, e.g. from U.S. Pat. No. 5,359,256. There, the drain of a field effect transistor (FET) is connected to a field emitter, and the current through the emitter is controlled by the voltage between the gate and the source of the FET.
The patterning of a specimen by means of an electron beam is usually performed by a scan where the beam current of electrons is switched on and off when it is directed from one spot on the specimen to the next. This requires the field emitter beam sources to generate fast beam current pulses in order to scan a specimen with high spatial resolution within a reasonable time. Therefore, the beam current pulses should have short rise and fall times in the range of a few nanoseconds or less in order to meet standard throughput requirements. However, with present field emitter beam source arrays that use current sources for beam current control, it is impossible to achieve such fast rise and fall times due to the inherently limited current of the current sources and the unavoidable parasitic capacitances, CP which are in parallel with the current source and which have to be charged and discharged for each beam current pulse.
As an example, the time Δt for charging a parasitic capacitance, Cp, of typically 10 fF with a current source providing a beam current, IE, of typically 10 nA to provide a switching voltage ΔU=5 Volt to switch on the beam current IE can be estimated to be Δt=5 μm, using the relationship Δt=CE ΔU/IE. Obviously, 5 μm is much too long for practical use. However, it is difficult to decrease the charging time, since it is difficult to (a) reduce the voltage ΔU, i.e. the voltage change necessary to switch on the beam current from zero to a beam current of a few nanoamperes; (b) reduce the parasitic capacitance Cp, which represents the stray capacitance of the emitter when it is connected with the current source; or (c) increase the current IE without worsening the focussing quality of the electron beam.